1. Field of the Invention
The present invention relates to positioning systems, fixed station apparatus, and positioning apparatus for detecting the position of mobile stations based on satellite transmission information that is transmitted from artificial satellites and DGPS data that is transmitted from fixed terrestrial stations.
2. Description of the Related Art
Global Positioning System (GPS) apparatuses are used to detect the absolute position (latitude, longitude, altitude) of moving objects and have become indispensable in many applications, such as, for example, automobile navigation apparatuses. GPS is used by moving objects to detect their position based on satellite transmission information that is transmitted from a plurality of satellites.
However, the position detection based on the information received by GPS apparatus has a position detection accuracy of approximately several hundred meters, which is not very precise. For this reason, differential GPS (DGPS) has been proposed for increasing the accuracy by virture of correction data, which reduces errors in the GPS positioning.
In DGPS, at a predetermined terrestrial location (i.e., fixed station) having an absolute position of confirmed accuracy, satellite transmission information is received from which the position of the receiver is detected (i.e., GPS positioned). By comparing the absolute position with the position obtained from GPS positioning, the error of the satellite transmission information is determined and correction data can, therefore, be supplied to the moving object to cancel the determined error.
The supply of correction data is performed through FM subcarrier broadcasts from FM broadcasting stations, where the FM subcarrier broadcasts have multiplex information (digital data) multiplexed in an ordinary 76 kHz FM broadcast band (a central frequency is 76 kHz). A moving object, such as a vehicle, receives the FM broadcasts on an FM multiplex receiver where the 76 kHz band signal is extracted from the received broadcast wave and is decoded to yield the multiplexed digital data. The correction data included in the multiplexed data is extracted, then used to correct the position information obtained at the GPS apparatus to yield position information having high accuracy. This sort of DGPS enables positions to be detected with an accuracy of a few meters.
There are instances where it is desirable to limit information provider services only to specific subscribing members. In this case, methods have been adopted which scramble information before it is provided. There scrambling methods render information completely unusable to those who are unable unscramble the information.
However, if the contents of services can be divided into levels according to the ranks of users, the degree of freedom in providing services increases so that services that meet the needs of users can be provided. For example, it would be convenient if the accuracy of correction data can be switched according to the rank of the user so that information meeting the rank of the user can be provided. In this manner, it is also possible to limit the positioning accuracy available to users when necessary.